The invention relates generally to semiconductor device fabrication and, in particular, to heterojunction bipolar transistors with a reduced base resistance, as well as fabrication methods for heterojunction bipolar transistors and design structures for BiCMOS integrated circuits.
Bipolar junction transistors are typically found in demanding types of analog circuits, especially analog circuits used in high-frequency applications. Bipolar junction transistors are found in radiofrequency integrated circuits (RFICs) used in wireless communications systems, as well as integrated circuits requiring high power efficiency, such as power amplifiers in cellular telephones, and other types of high speed integrated circuits. Bipolar junction transistors may be combined with complementary metal-oxide-semiconductor (CMOS) field effect transistors in bipolar complementary metal-oxide-semiconductor (BiCMOS) integrated circuits, which take advantage of the favorable characteristics of both transistor types.
Conventional bipolar junction transistors include three semiconductor regions, namely the emitter, base, and collector regions. Generally, a bipolar junction transistor includes a pair of p-n junctions, namely an emitter-base junction and a collector-base junction. A heterojunction bipolar transistor (HBT) is a variety of bipolar junction transistor that employs at least two semiconductor materials with unequal band gaps for the emitter and base regions, creating a heterojunction. For example, the base of a HBT may be comprised of silicon germanium (SiGe), which is characterized by a narrower band gap than silicon typically composing the emitter of the HBT.
Improved devices are needed for HBT's that boost device performance, as well as improved fabrication methods for HBT's and design structures for BiCMOS integrated circuits.